PHA is a non-specific stimulant of normal lymphocytes resulting in vitro in their blast-like transformation with increased RNA and DNA synthesis, mitosis, and the release of putative mediators of cell-mediated immunity. Histologically the skin reaction to intradermal PHA shows perivascular infiltration of mononuclear cells, the hallmark of delayed-type hypersensitivity. Since prior exposure is not required—only the capacity to respond—PHA appears to be the ideal agent to assess cell-mediated immunity in vivo. In a continuing series of more than 30 individuals 2 μg of PHA intradermally produces erythema and induration in 24–48 hrs., despite the presence or absence of cutaneous reactivity to other skin-test antigens (PPD-S, histoplasmin, mumps, SK-SD, candida, and trichophyton). In a selected group of patients in vivo skin reaction to PHA correlates with in vitro response of the individual's peripheral blood lymphocytes to PHA. PHA in vivo thus appears to be an effective measure of intact cell-mediated immunity and is especially useful in infants and children who have had limited prior exposure to those antigens normally used as skin-test reagents.